1. Field of the Invention
The present invention relates to a failure detecting system of an exhaust purification apparatus, having a catalyst unit, inclusive of an electrically heated catalyst, that is inserted in an engine exhaust path and an air pump for supply of secondary air to the exhaust path in such a manner that current is supplied to a structure as the electrically heated catalyst and the air pump, upon cold start of the engine.
2. Description of the Related Art
Conventionally known as this kind of failure detecting system is an electric current sensor for detection of current value supplied to the electrically heated catalyst and an electric current sensor for detection of current value supplied to the air pump, wherein presence of failures in the electrically heated catalyst and air pump are discerned on the basis of electric current value detected by these electric current sensors (See Japan Patent Unexamined Publication No. Hei. 9-60554).
Immediately after the air pump is started, the electrical current increases temporarily under the influences of inertia mass and startup character of the air pump, and stabilization of the electric current takes a certain duration of time. Therefore, in the above-described conventional type, after the start of supply of current to the electrically heated catalyst and the air pump, a certain duration of time has been retained for a waiting time as required to stabilize the electric current for the pump, and then the detection of electric current value for the electrically heated catalyst and the detection of electric current value for the air pump are simultaneously started.
In order to secure durability of the electrically heated catalyst, supply of current to the electrically heated catalyst is stopped if the engine rotation frequency or vehicle speed exceeds a predetermined value even at the cold start. If a driver, however, attempts to start driving a car immediately after beginning of the startup, supply of current to the electrically heated catalyst is stopped because the engine rotation frequency or vehicle speed has exceeded the predetermined value before expiration of necessary duration of detection time after the start of detecting the electric currents for the electrically heated catalyst and the air pump, and thus discernment of failures of the electrically heated catalyst cannot be carried out.
Further, a conventionally known engine exhaust purification apparatus is provided with a catalyst unit which contains an electrically heated catalyst that is inserted in the engine exhaust path. The electrically heated catalyst is supplied with current upon cold start of the engine to facilitate exhaust purifying reaction in the catalyst unit for improvement in the exhaust emission at the cold start (Japanese Patent Unexamined Publication No. Hei. 9-21313).
Also known is the one provided with a failure detecting means in which, after detection of current value supplied to the electrically heated catalyst, an integrated power amount that has been applied to the electrically heated catalyst is calculated on the basis of the detected electric current value so as to be utilized as a parameter representing the generated heat amount of the electrically heated catalyst, and an electrically heated catalyst failure is discerned if the integrated power amount is not within a predetermined allowable range.
If the electrically heated catalyst is supplied with current when the engine temperature is substantially lower than the normal temperature, a heat shock which incurs bad influence in the durability thereof is liable to occur in the electrically heated catalyst.